1. Field of the Invention
This invention is a new and novel ring laser gyroscope. It utilizes stimulated Brillouin scattering (SBS) in an optical fiber waveguide to produce two independent, counterrotating laser beams that are heterodyned together to sense rotation. The common cavity for these two laser beams is the optical fiber waveguide that is wound on a form to enclose an area, A. When the two beams are heterodyned together a beat frequency is obtained that is directly proportional to the enclosed area, A, and to the rotation rate, .OMEGA.. The acousto-optical effect called stimulated Brillouin scattering is used to produce the two ring lasers operating respectively in the clockwise and counterclockwise directions around the optical fiber cavity. Each Brillouin ring laser (BRL) is pumped by a pump laser beam derived from an external pump laser. An important, novel, and central feature of this invention is that the two pump beams are operated at different frequencies. Difficulties inherent in other laser gyroscopes are negated by this technique, and a simpler, more versatile, more accurate system for rotation rate readout is achieved.
2. The Prior Art
Conventional ring laser gyroscopes utilize optical oscillators (lasers) to produce rotation sensors that can be made into rate integrating gyroscopes. In practice a ring laser consists of a gain section and two or more passive sections. The ring is optically closed on itself by means of mirrors, and the closed ring containing the gain section constitutes the laser cavity. Two coherent laser oscillations are established within the cavity, one in the clockwise (CW) and one in the counterclockwise (CCW) direction around the closed ring. A portion of each circulating beam is split off by a beam splitter and these two split off beams are recombined and heterodyned together to produce a beat frequency. The beat frequency is directly proportional to the area enclosed by the ring and to the rotation rate about any axis perpendicular to the plane of the ring. The beat frequency can be expressed mathematically by the equation: ##EQU1## In Equation 1, .DELTA.f is the beat frequency in Hertz (Hz), A is the are enclosed by the ring in square meters (m.sup.2), .OMEGA. is the rotation rate about any axis perpendicular to the plane of the ring in radians per second (rad/sec), .lambda. is the wavelength of the laser radiation in meters (m), and L is the cavity length in meters measured along the beam path (the same for both beams).
All optical rotation sensors can be explained theoretically in terms of the Sagnac effect. Sagnac showed in 1913 that it is possible to detect rotation optically. In his instrument (a ring interferometer), and area, A=866 cm.sup.2, was circumscribed by oppositely traveling beams of light. The two beams were combined to form an interference pattern. A rotation of the device produced a fringe shift. The fringe shift was directly proportional to the rotation rate.
Subsequent analyses based on both relativistic and non-relativistic arguments led to the same conclusion; namely, that the transit time around the ring for light traveling in the direction of rotation is different from that for light traveling opposite to the direction of rotation. The time difference is given by ##EQU2## and the corresponding effective path length difference is given by ##EQU3## In Equations 2 and 3, .DELTA.t and .DELTA.L are the effective transit time and length differences, respectively, and c is the free space seed of light. A is the area enclosed by the ring and .OMEGA. is the rotation rate, both as in Equation 1.
The salient features of the Sagnac effect have been shown to be:
(a) Equations 2 and 3 are correct.
(b) The results are independent of the shape of the area, A.
(c) The results do not depend on the location of the axis of rotation.
(d) The results do not depend on the presence of a co-moving refracting medium in the beam path.
It is important to note that a measurement of the optical path length difference (Equation 3) enables an observer located on a rotating frame of reference to measure the absolute rotation of his frame in inertial space.
The basic requirement for coherent laser oscillation within a closed ring laser cavity is that the cavity length L, and the laser wavelength, .lambda. as measured in the cavity must satisfy the relationship EQU L=m.lambda. (4)
where m is an integer. Equivalently, the laser oscillation frequency corresponding to this wavelength is EQU f=mc/L (5)
where f is the laser oscillation frequency and the other terms are as previously defined. Typically, m is a large number in the range from 10.sup.5 to 10.sup.7 ; or larger, if the cavity is a long optical fiber.
Equations 4 and 5 do not imply that oscillation is impossible unless some very stringent condition is met relative to cavity length and laser wavelength. What is implied is that the laser oscillation frequency will adjust to a value where the equations are satisfied for some integer value of m. It is assumed, of course, that system gain and gain-bandwidth are adequate to permit oscillation. Simple manipulation of Equations 4 and 5 leads to the result ##EQU4## where the minus sign indicates a decrease in frequency for an increase in L. Combination of the first and last terms of Equation 6 with Equation 3 leads immediately to the result ##EQU5## This is identical to Equation 1 and is the basic laser gyroscope equation relating beat frequency, area, rotation rate, wavelength, and cavity length.
Reduction to practice of the theoretical possibilities embodied in Equation 7 has proceeded along two general lines. The first approach employs a gaseous helium-neon laser gain section and, typically, two passive sections. The three sections are arranged in an equilateral triangle configuration. Mirrors at the triangle vertices close the ring. Two counterrotating laser oscillations are maintained in the ring and operation is basically as predicted by Equation 7. This device was disclosed in U.S. Pat. No. 3,484,169. However, there are undesirable effects operating in any such device. These effects can be classified as null shift, lock-in, and mode pulling.
Null shift errors are present whenever the cavity is anisotropic with respect to the CW and CCW laser beams, that is, when the effective cavity length is different for the two directions. In that case the oscillation frequencies are different for the two beams even when the rotation rate is zero. This beat frequency in the absence of rotation (null shift) can be a very significant source of error. In deriving Equation 7, the geometrical cavity length, L, was used. In an exact analysis the optical path length is required, which is the integral of refractive index over the path length. Any effect that causes the refractive index along the path to be different for the CW and CCW directions will introduce a null shift. One such effect is the Fresnel-Fizeau drag effect. It has been shown that the velocity of light, .nu., in a moving medium whose index of refraction is n and whose velocity is V is given by ##EQU6## where the plus and minus signs correspond, respectively, to light traveling in and against the direction of the motion. In Equation 8 the velocity of light in the medium, .nu., has changed from its nominal value of c/n and an anisotropy has been introduced relative to the two directions of travel.
The electric discharge in the gaseous gain section of a ring laser imparts an organized flow velocity to the electrons and positive ions in the discharge plasma. In addition, a net flow of neutral gas atoms is also induced (cataphoresis). Each of these flows contributes to the Fresnel drag effect discussed above and, therefore, to a null shift. Also, any air flow within the passive sections of the cavity would contribute to a null shift. Such effects can produce beat frequencies orders of magnitude larger than those due to typical rotational inputs. There are other lesser sources of null shift such as magnetic fields and nonreciprocal saturation effects in the gain medium.
Lock-in is a well known phenomenon common to all coupled oscillator systems. The bidirectional ring laser is such a coupled system. There is mutual coupling between the couterrotating oscillations. Consequently, at low rotation rates where the oscillations are at very nearly the same frequency, they lock together. Thus, a dead band is produced wherein the system is not responsive to rotation. The dominant source of coupling between the two oscillations is the mutual scattering of energy from each beam into the direction of the other. This is mainly due to scattering from the mirror surfaces. Even with optimum design the lock-in phenomenon is invariably present in ring laser gyros with gaseous gain sections.
Mode pulling is the third major source of error in a conventional ring laser gyroscope. In the derivation of Equation 7, the optical gyroscope equation, an ideal, empty cavity was assumed. However, the actual active gain section contains the lasing medium that is the source of the laser radiation. Any such medium is dispersive, that is, its refractive index varies with frequency. Dispersion is classified as normal in frequency regions where the refractive index varies slowly and smoothly with frequency. However, in regions near a resonance (gain maximum) the index undergoes a rapid change and there is absorption. This is referred to as anomalous dispersion. Because of anomalous dispersion effects, an oscillating mode will change in frequency from the value predicted by empty resonator theory (mode pulling). Alternatively, two modes oscillating with nearly equal frequencies can pull closer together, the extreme case being lock-in. The frequency shift caused by mode pulling causes a change in the gyro scale factor and leads to an error in rotation rate readout.
All of the difficulties detailed above with respect to conventional ring laser gyros have been resolved to some degree of satisfaction. Techniques have been developed to defeat or compensate the major error sources. For example, proper design of the laser will guarantee single mode operation and eliminate low frequency noise caused by mode coupling. Operating at low gain minimizes mode pulling and leads to a reasonably stable scale factor. Static null shifts are minimized by keeping non-reciprocal elements out of the optical path. Two balanced DC discharges are used to cancel null shifts due to Langmuir flow effects. The entire cavity is sealed to prevent null shifts due to air flow in the beam path. Lock-in thresholds are minimized by careful mirror design.
Although lock-in cannot be completely eliminated, the problem can be made tractable through deliberate imposition of a known null shift bias. This bias provides a beat frequency in the absence of rotation that is known and that can be subtracted from an apparent rotation readout to obtain the true rotation rate. Null shift biases have been generated by several means. Included are discharge gas flow, Langmuir flow, Faraday effect, and mechanical motion of the gyro. The preferred method is alternating mechanical motion (dithering).
Useful discussions of basic theories involved in the conventional laser gyro can be found in IEEE SPECTRUM, "The Laser Gyro" by Joseph Killpatrick, October 1967, pages 44-45; and in the book Laser Applications, edited by Monte Ross, Academic Press, New York, 1971, "The Laser Gyro" by Frederick Aronowitz, pages 133-200.
A second approach to ring laser gyroscope design is to use a solid laser gain section such as neodymium-yttrium-aluminum garnet (Nd:YAG) or ruby. Unfortunately, all solid lasers such as Nd:YAG or ruby exhibit homogenous line broadening. It is not possible to maintain two oppositely directed, nearly equal frequency, independent oscillations in such a gain medium. The possibilities have been thoroughly investigated. However, it has not proved feasible to use solid laser technology in ring laser gyroscopes.
A third approach to ring laser gyroscopes uses optical fiber as the laser cavity and depends on stimulated Raman scattering or stimulated Brillouin scattering to produce the required ring lasers. Each of these processes depends on an external pump laser to excite and sustain the internal oscillations in the optical fiber ring. One such device was disclosed in U.S. Pat. No. 4,159,178. This device has not been reduced to practice and analysis indicates it may be very prone to lock-in with lock-in thresholds too high to be compensated by practical means.
In view of the foregoing discussion relative to conventional laser gyroscopes and solid and optical fiber gyroscopes disclosed to date, it is clear than an improved laser gyroscope would be highly desirable. In particular, any genuine improvement relative to the major sources of error in laser gyroscopes, namely, null shift, lock-in, and mode pulling would be of technological advantage and importance. The invention disclosed and claimed herein is a novel, new ring laser gyroscope that has advantages over all laser gyroscopes previously disclosed.